They haven't decided on that yet. They have the option to mothball it for extended periods, as long as occasional maintenance visits are made. Apparently (according to CNN) they were considering doing this for part of this year anyway. CNN didn't explain why they were considering this though, perhaps they are waiting from another module from the Russians or something ?

I'm pretty sure that techically they could keep it running without the Shuttle (by paying the Russians extra), but adding new modules would be next to impossible I suspect.